The present invention relates to a rear derailleur for bicycle gears, which is adapted to derail the bicycle chain on to a series of sprockets of increasing diameter carried by the rear hub of the bicycle. The invention concerns, in particular, a rear derailleur of the type comprising:
a support body for connection to the bicycle frame,
a rocker arm which carries idler wheels for the chain and which is mounted on the support body for sliding movement along an axis parallel to the axis of the rear hub of the bicycle between a first end position in which the chain is engaged with the smallest-diameter sprocket and a second end position in which the chain is engaged with the largest-diameter sprocket, the rocker arm being pivotable about the axis parallel to the axis of the rear hub of the bicycle,
resilient means for urging the rocker arm towards the first position and for urging the rocker arm to pivot about the axis in the sense corresponding to the tensioning of the chain, and
means for moving the rocker arm towards the second position. A derailleur of this type, described in French Pat. No. FR-575,191 has the disadvantage that it can be used to advantage only on bicycles of an age which are provided with no more than three sprockets keyed to the rear hub, corresponding to three different speed ratios. The example described in the above French patent does not enable the chain to be derailed on to the idler wheels of modern touring or racing bicycles with seven sprockets, since the spring with which the derailleur is provided, by becoming packed, does not allow the rocker arm to travel a sufficient distance. It might be possible to increase the travel of the cylindrical spring but this would require the use of a rocker arm of excessive width which would project too much from the bicycle. Moreover, the pivoting of the rocker arm is not sufficient to take up the portion of the chain which becomes free when the chain moves from the smallest sprocket or ring to those with greater numbers of teeth.